Diametral Tensile Strength, Microhardness, Surface Modulus, and Surface Morphology of Novel,Antibacterial Orthodontic Adhesives.

Abstract

Objective: Prior to the advent of bonded orthodontic appliances each tooth was fitted with a band made from stainless steel. Traditionally they were cemented to the tooth with a zinc phosphate cement. This acted more as a luting agent then adding to the actual adherence of the band to the tooth. In addition, often times the cement would wash out and upon the band removal white spot lesions and or carious lesions were present. The development of glass monomer cements had a traumatic improvement over both the adhesion to the tooth and a diminishing of white spot lesions in part due to the release of fluoride. Since the advent of the acid-etch technique and the bonding of orthodontic brackets directly to the enamel various bonding adhesives were developed. The first and most popular bonding resins were chemical curing bonding systems. The general class of self-adhesive and/or self-etching orthodontic adhesives are of several types, including resin-modified cements, such as resin-modified glass ionomer cements, that exhibit self-adhesive properties to enamel, as well as self-etching primers that combine the conditioning and priming agents into one acidic, polymerizable composition for application to the tooth surface, and do not require separate etching and rinsing steps. Not only do resin modified glass ionomers have the benefit of chemically bonding to a clean and un-etched enamel surface, but these self-adhesive cements also release significant and continuous levels of fluoride ion. Recently investigators have explored adding antimicrobial agents in addition to fluoride in orthodontic adhesives. One such approach has been the addition of ZnO powder, a known compound with anti-microbial properties, to an orthodontic resin modified glass ionomer adhesive. It should also be noted that zinc ions and compounds have been shown to positively influence enamel remineralization and enhance apatite formation. The objective of this research is to evaluate the effect of the addition of an antimicrobial agent, zinc oxide powder, on selected mechanical properties of a resin modified glass ionomer orthodontic adhesive. Methods: A resin modified glass ionomer (Fuji Ortho LC), in its two-component, powder/liquid format were utilized in this study. Control specimens, according to manufacturer's directions for use, were prepared for both diametral tensile and microhardness testing with be prepared. Using the powder component provided in the marketed product; powder blends containing both 10 and 20 percent Zinc Oxide (by weight) will be added to the RMGI powder, and thoroughly mixed and blended to create a uniform powder blend. The mechanical testing will be performed on 8-10 disc specimens, approximately 6.2 mm diameter by 3.1 mm in height, using a standardized mixing and molds for each material. The specimens will be tested using the Instron 5569 testing machine at a crosshead speed of 0.75 mm/minute for DTS until failure occurs. Data was recorded in Newton’s (N) force. The microhardness testing was performed on 12 disc specimens, approximately 6.2 mm diameter by 3.1 mm in height, using a standardized mixing and molds for each material. A CSM microindentation testing device was used to measure theVickers microhardness. The surface morphological evaluation of the specimens both control and experimental will be examined at 50 X magnification for comparison of surface characteristics and morphology. One-way ANOVA for comparison of time- and material-specific mean Vickers microhardness values and post hoc pair-wise comparisons was employed to assess statistically significant differences in the mean values (p<0.05). Results: The diametral tensile strength test of all specimens including the modified control and experimental showed incremental decreases in the DTS as compared to the control mixed as per the manufacturer’s specifications. The Vickers values illustrated minimal variation of Vickers microhardness for the control and experimental group. The surface morphological evaluation illustrated various differences between the control, modified control and the Zinc Oxide formulations. Conclusions: Alterations in the liquid powder ration of the orthodontic resin modified glass ionomer adhesive resulted in a reduction of the DTS. The addition of both 10% and 20% zinc oxide powder also resulted in a significant reduction of the DTS as compared to the manufacturers mix proportions of liquid and powder. The Vickers microhardness did not illustrate a significant alteration in any of the specimens. The addition of both 10% and 20% zinc oxide powder to the mixture reduced the modulus and stiffness as compared to both of the control groups. Lastly, the morphology of the experimental samples with the zinc oxide showed a more irregular surface at the fracture site.